The invention relates to a method for cold starting the internal combustion engine of an automobile. In general, the goal of the invention is to reduce at the origin the polluting emissions of gasoline engines.
The quality of fuel used for vehicles varies greatly, especially as a function of the geographical zone where the vehicles operate. A particularly variable physical property of fuel is its vaporizing capacity, in other words its varying volatility. This capacity is well known in Anglo-Saxon literature under the acronym RVP (Raid Vapor Pressure). This acronym will be used in the following description of the invention. Fuels that vaporize easily are called HRVP (High RVP) and fuels that do not vaporize easily are called LRVP (Low RVP).
In order to start correctly, a gasoline engine requires a mixture of air and gasoline close to the stoichiometric mixture. This assumes proper control of the quantity of fuel under gaseous form. According to the volatility of the fuel, the quantity of fuel under gaseous form that participates in the combustion during cold start and when the engine is cranked can vary enormously for the same quantity of injected fuel.
In order to ensure a sufficient quantity of fuel under gaseous form for proper combustion during start and cranking of the engine, calibrations are made with a fuel that is representative of a fuel with relatively low volatility (LRVP). Then, tests are performed to ensure that when a more volatile fuel is used, type HRVP, the injected quantities are not excessive and there is no risk that excess gasoline in vapor form will hinder the combustion, due to the mixture becoming non-inflammable.
Therefore, the adjustment is the same regardless of the fuel. Consequently, when a relatively more volatile fuel is used, the quantity of fuel in vapor form is excessive during start and cranking of the engine. This excess does not participate in the combustion and is found in the exhaust of the engine in the form of unburned hydrocarbons (HC). This has a direct impact on the polluting emissions of the engine because even if the vehicle is equipped with a catalyst, the catalyst is not cold primed and the unburned hydrocarbons escape to the atmosphere.
During start in extreme cold, when the ambient temperature is below −15° C., the excess fuel in vapor form also creates black smoke at the exhaust.